Formal Assertions

Formal assertions spell out and enforce the contract among a module and its clients. Conceptually, formal assertions are constituent of the stipulation of a module, no dissimilar from stating which data forms are to be passed as parametric quantity or brought back by a function.

ñ  Just as the compiler enforces which certain forms bear to be passed as parametric quantity and brought back at compile time;

ñ  And (in C++), the linker effectively enforces the conformance of library functions to their stated interfaces (subdivision-safe linkage);

ñ  Formal assertions point out violations of the computer programming contract at execute-time.

When the module is compiled with formal assertions enabled, the module halts the program with a meaningful content if either the module or one of its clients fails to fulfill its obligations.

Require and Ensure

Preconditions and post considerations are grouped together into clauses. Were it possible for software developer to transplant programming language -level corroborate for formal assertions onto C or C++, the require clauses would look like constituent of the function prototype, and might look something like this:

   require {

     expr;

      expr;

   }

By bad luck, software developer cannot do this as graciously as software developer would like without programming language -level corroborate. Thus, software developer produced preprocessor macros X ensure and require to copy the Eiffel keywords of the similar name as intimately as possible, and to render aims for the clauses  where they are most seeable, at the cover of the function definition. Major feature of Assertions are:

ñ  Assertions are boolean expressions which serve to express the semantic properties of classes and member functions.

ñ  # Assertions are similar to the mathematical notion of a predicate.

ñ  # Assertions are tools for expressing and validating the correctness of modules, classes and subprograms.

Significant functions of Assertions

ñ  Support in constructing correct programs. For example,  it determine input preconditions and output postconditions.

ñ  Documentation corroborate. For example,  it aids in computer programming by contract

ñ  Debugging aid.  For example,  it  find out where  or when assumptions are not correct.

ñ  Basis for an exception mechanism. For example,  it  integrate with exceptions by permitting assertion failures to be caught dynamically.

Types of Assertions

ñ  Assertions are employed for various functions:{ Preconditions

ñ  State the requirements under which sub- programs are applicable.{ Postconditions

ñ  Properties guaranteed upon subprogram expire.{ Class Invariants# Properties which characterize class illustration s across their lifetime.  Subprogram preconditions and post-

considerations are implicitly assumed to comprise the class invariant. { Loop Invariants

ñ  Loop invariants determine properties which are always true during the execution of a loop.

Using Assertions to Specify ADTs

Conceptually, ADTs comprise of four constituents:

(1) forms

(2) functions

(3) preconditions/postconditions

(4) axioms

In whatever means , most languages only permit stipulation of the First two constituentst (i.e.functions and forms). Assertions render a mechanism to express the preconditions and axioms corresponding to ADTs.

Formal Assertion

A formal assertion is an assertion which has a prescribed formal relationship to the semantics of a function. Preconditions and postconditions are examples of formal assertions. Put in more contract-like terms, they constitute disclaimers and guarantees, respectively.

Invariant of Assertion

An invariant is an assertion which describes properties of a data structure which never modify. When enabled, class invariants are checked upon entering and exiting every one considered individually method. If an invariant is violated upon entering a method, then the integrity of which illustration  was violated prior to calling the method. If the invariant is violated upon exiting a method, then the method did not fulfill its obligation to go forward the integrity of the object illustration .

Postcondition of Assertion

A postcondition is a formal assertion  which expresses a obligation of the function to fulfill. A postcondition violation points which either the function does not exercise correctly, or which the assertion is incorrectly assigned. Postconditions assume which all preconditions bear been satisfied.

Precondition of Assertion

A precondition is a formal assertion which expresses a obligation of the client which bear to be fulfilled prior to calling the function for the function to operate correctly.

Restrictions of Assertion

Some characteristics are best rendered at the programming language  level. They are far more gracefully comprised which means. Software developer bear to do the best which software developer could, all the same, where essential programming language  characteristics are missing.

ñ  CrFormal.h employs the require and ensure clauses as opportunities to do class invariant assuring. Where the clauses are missing, invariants likewsie cannot be checked.

ñ  The ensReturn(subdivision, expr) and ensReturnVoid macros could only be employed at one time in any function. Neither could be employed in absence of an ensure clause. This does not pose a issue for those whose convention it is to render only a single expire point for any function. It is almost of the time not a good practice to rely on computer programming conventions when adding programming language -like characteristics. At least in this event, a compiling error occurs if a construct is missing or employed too various times. There is no protection from raw bring back statements in addition to a ensReturn statement, all the same.

Two versions of the ensReturn (subdivision, expr) macro are comprised in CrFormal.h. One employs a temporary variable of the assigned subdivision to prevent the expression from being judged two times.

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